During sleep, all muscles, including those of the upper airway, lose tone and relax. Obstructive Sleep Apnea (OSA) occurs when tissue blocks the upper airway during sleep. This will cause a drop in blood oxygen and a rise in blood carbon dioxide. The brain will sense these changes, and awaken the person enough to restore muscle tone to the structures of the upper airway, and the airway will reopen.
The severity of OSA is determined by the number of blockages per hour of sleep, also called the apnea-hypopnea index (AHI). These include complete blockages (apneas) and partial blockages (hypopneas). The severity of OSA, as determined by a sleep study, is classified as follows:
SeverityBlockages per Hour (AHI)Mild 5-15Moderate15-30Severe30+
OSA disrupts restorative sleep. Chronic fatigue has long been recognized as the hallmark of OSA. But more recently, large clinical studies have shown a strong link between OSA and stroke and death. This link is independent of other risk factors for cardiovascular disease such as hypertension, obesity, high cholesterol, smoking and diabetes.
Current Therapies
Several structures can cause blockage of the upper airway: the tongue, the soft palate, the lateral walls of the pharynx, the tonsils and the epiglottis. In most patients, the blockage is caused by a combination of these anatomical structures.
Many procedures and devices have been used to stabilize, modify or remove tissue in the airway to treat OSA. In uvulopalatopharygoplasty (UPPP), the uvula, part of the soft palate and the tonsils are removed. The Repose stitch is used to tie the tongue to the mandible to prevent its posterior movement. Oral appliances move the mandible forward (very slightly) to create more space in the airway.
None of these approaches has achieved much more than a 50% success rate, with success defined as a 50% decrease in AHI to a score below 20. The limited success of these approaches likely stems from the fact that they don't address all anatomical sources of a blockage.
The most widely used therapy for OSA is Continuous Positive Airway Pressure, or CPAP. A CPAP system consists of three parts: an airtight mask fitting in or over the nose or nose and mouth, an air pressurizing console and a tube connecting the two. The mask contains one or more holes. CPAP works by pressurizing the upper airway throughout the breathing cycle, essentially inflating the airway to keep it open. CPAP thus maintains a pneumatic splint throughout the respiratory cycle.
Unlike interventions that treat specific blockages, CPAP addresses all potential blockage sites. The success rate in patients exceeds 80%, and its cure rate (decreasing AHI below 5) is close to 50%. The drawback to CPAP is poor patient compliance. Roughly half of all patients who try CPAP are unable to sleep with it. Patients dislike several aspects of CPAP including: having to wear a mask, being tethered to a pressurizing console, the noise of the console, traveling with a bulky device, and a loss of personal space in the bed.
There is good evidence that an effective pneumatic splint can be achieved within part of the respiratory cycle by producing a partial blockage in the nose or mouth, thus slowing the release of air during expiration. The simplest method, pursing of the lips, has been shown to open the upper airway and improve breathing in emphysema patients.
Doshi et al. (US Patent Application 2006/0150978) describe removable nasal devices that provide considerably more resistance during exhalation than during inhalation. Early results with this type of device are promising, although the results are not as good as those achieved with CPAP. See, Colrain I M, Turlington S. The use of a nasal resistance valve to treat sleep disordered breathing. SLEEP abstract 2008; Rosenthal L, Dolan D C, Massie C A, Kram J. A novel expiratory pressure device to obstructive sleep apnea. SLEEP abstract 2008; Massie C, Rosenthal L, Krarn J. Acceptance and Adherence of a novel device in the treatment of sleep apnea. SLEEP abstract 2008.
The drawback to the devices described by Doshi is that increased airway pressure (the “Pneumatic splint”) is only achieved during exhalation: there is no increased pressure during inhalation. Additionally, the nasal device described by Doshi cannot be used beneficially by mouth breathers, or patients who become mouth breathers when resistance is added to the nasal passages.
Several devices providing a proximal blockage and covering both the nose and mouth have been described. Oren (U.S. Pat. No. 5,649,533) describes a mask covering the nose or nose and mouth which has two valves. The first valve opens during inhalation, that is when external pressure exceeds pressure within the mask. The second valve opens when pressure within the mask exceeds pressure outside the mask within a certain range, but which will close when pressure within the mask exceeds atmospheric pressure by a predetermined amount (as would be achieved near the end of expiration). This device thus relies on complete closure of all valves near the end of expiration to achieve a pneumatic splint. The drawback to the system is that it does not allow the patients to complete expiration before initiating inspiration.
Bibi (U.S. Pat. No. 6,371,112) describes a system that contains both a mouthpiece and a nasal mask. This fairly complex system uses an inflatable body within the mouthpiece to maintain elevated pressure within the airway throughout the respiratory cycle. The drawback to the system is the requirement for a sizable device within the mouth.